A technique is disclosed in Tokkai Hei 7-144119 published by the Japanese Patent Office in 1995 for the reduction of hydrocarbon (hereafter HC) emissions immediately after cold-starting an engine at which time a three-way catalyst disposed upstream has not reached an activation temperature. According to the technique, a composite catalyst, which is comprised of a three-way catalyst and an HC absorbent material and which allows for the absorption and release of HC depending on the temperature of the catalyst, is provided downstream of a three-way catalyst.
A composite catalyst enables the temporary absorption of HC exhausted immediately after cold-starting an engine. Thereafter when the upstream three-way catalyst reaches an activation temperature, the air-fuel ratio is varied to a lean air-fuel ratio resulting in a super-oxygenated air mixture which raises the catalyst temperature of the composite catalyst. As a result, absorbed HC is released and oxidized on the composite catalyst.